Dynamic Compression Ratios... and blown engines

[06Speed6]

Ive been doing some pretty intensive research into cam specs, stroke, rod length, and bore diameters and their effects on engine performance and how they all tie into each other.... so I can build an engine that will make enough hp to bitch smack all you naggahs back to 1983.

An explanation of Dynamic Compression Ratio (DCR) vs Static Compression Ratio (SCR): SCR is calculated from total amount of compression available from the measurement beginning of Bottom Dead Center (BDC) which is the maximum amount of compression mechanicly available. DCR is calculated on the compression stroke after the intake valve closes since you cant really have compression with the valves open. The DCR will always be less than the SCR and the DCR is closer to a real world compression ratio.

The only difference between DCR and SCR is that on the DCR the intake valve closing is calculated into the equation.


My engine specs with aftermarket cam:
Bore = 87.5mm/3.445"
Stroke = 94mm/3.701"
Rod c/c = 154.75mm/6.092"
Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 115*
Inlet Valve Closes After Bottom Dead Center (ABDC): 57.5* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.00:1
Dynamic Cranking Pressure: 137.81 PSI
Dynamic Boost Compression Ratio: 17.80:1


The dynamic compression ratio for this setup with my aftermarket cams is 8.0:1 which is the middle of the acceptable range of 7.5:1 and 8.5:1 for pump gas engines. Anything less than 7.5:1 is bad for power and torque and anything above 8.5:1 will generally knock on pump gas.

If we change only the LSA to the factory cam spec of 108*, we get:
Bore = 87.5mm/3.445"
Stroke = 94mm/3.701"
Rod c/c = 154.75mm/6.092"
Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 108*
Inlet Valve Closes After Bottom Dead Center (ABDC): 54* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.19:1
Dynamic Cranking Pressure: 141.89 PSI
Dynamic Boost Compression Ratio: 18.22:1


Now to Lex's theory, which I generally laughed at: He stated that low rpm torque lead to grenaded engines and the boost and timing ranges needed to be altered to move the peak torque higher in the powerband.

I knew Lex was onto something but I also though he was wrong because we have a very long stroke and if our rods were that weak the piston velocities would be grenading engines left and right at high rpm and high boost long before the low rpm torque would do anything.

Here is something that struck me today when I was running the numbers and expectially after reviewing Subparpunk03's VVT chart. The variable valve timing advances the intake cam a max of 18* in relation to the exhaust cam, in the higher load ranges of RPMs less than 5000. This cam advance does two things, it moves the powerband of the cam lower so we can have both a low rpm high torque cam and a high rpm high hp cam, but here is the catch: it raises the dynamic compression ratio of the engine in low RPMs to potentially dangerous levels that could induce knock.

If we advance the LSA of the 108* cam 18* we get:
Bore = 87.5mm/3.445"
Stroke = 94mm/3.701"
Rod c/c = 154.75mm/6.092"
Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 90*
Inlet Valve Closes After Bottom Dead Center (ABDC): 45* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.58:1
Dynamic Cranking Pressure: 150.3 PSI
Dynamic Boost Compression Ratio: 19.09:1

If we advance the LSA of my 115* cam by 18* we get:
Bore = 87.5mm/3.445"
Stroke = 94mm/3.701"
Rod c/c = 154.75mm/6.092"
Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 97*
Inlet Valve Closes After Bottom Dead Center (ABDC): 48.5* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.44:1
Dynamic Cranking Pressure: 147.28 PSI
Dynamic Boost Compression Ratio: 18.77:1

This dynamic loading could explain why our engines like to knock and grenade at low rpms and almost no one blows at high rpm or high boost. The bottom line is that the closer you get to 8.5:1 dynamic compression the more likely you are to knock and engine reliability goes down the drain. At around 8.58:1 near idle coupled with very advance timing and close to stotch a/f I think we could be walking a razors edge for low rpm knock even with the benefits of direct injection.

One final thought after sleeping on everything last night, if we look at that chart and take it as a percentage of the whole 18* of maximum advance, then 30 on the chart would be 30% of 18* which is exactly the nice round number 8.30:1 dynamic compression as evidenced by the following:

Bore = 87.5mm/3.445"
Stroke = 94mm/3.701"
Rod c/c = 154.75mm/6.092"
Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 102.6*
Inlet Valve Closes After Bottom Dead Center (ABDC): 51.3* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.30:1
Dynamic Cranking Pressure: 144.26 PSI
Dynamic Boost Compression Ratio: 18.46:1


The bonus to all of this is that with the proper aftermarket cam, we can have our cake and eat it too because we can run an agressive, lopey cam and still enjoy a smooth idle.

Special thanks to Subparpunk03 and Superskaterxes for helping me with this.

What are your thoughts?

[subparpunk03]

I'm not understanding something...

Am I reading that correctly that adding 30* to the intake cam advance means lowering the static compression ratio?

I think I'm missing something here, can you take a step back and for my benefit explain how the intake cam advance effects the dynamic compression ratio with all other factors unchanged?

[06Speed6]

The valve is open longer with the extra 30* of intake cam timing so the piston is higher in the bore by the time the intake valve closes. Increasing the lobe seperation angle allows more air into the engine at higher rpms, but lowers the effective compression ratio because you cant compress the air in the cylinder if the intake valve is open.

On a normal engine the intake valve will always be open for the first part of the compression stroke. Dynamic compression ratio starts the calculation once the piston reaches a point in the bore where the intake valve is closed and actual compression can begin. Static compression ratio starts the calculation from the very bottom of the bore.

The static compression ratio is unchanged.

[nighthawk358]

damn...we need more people like u 06speed6

[subparpunk03]

My bad, I meant DYNAMIC compression ratio... so more increase in the intake cam means LESS dynamic compression... so we are running more increased VVT in the mid-load/part-throttle area, so wouldn't lowering the dynamic compression there be a good thing?

and does the VVT increase the length of time that the intake cam is open, or just change the time at which the valve begins to open? If the valve opens earlier but stays open for the same length of time, I am picturing it in my head that increasing intake cam advance would increase the dynamic compression ratio... I'm still confuzzled.

[itzl0l]

so....ok......what do we do about this to keep the CEL's (change engine lights) at bay?

[phantom3]

So if I understand this correctly, we could benefit from a VVT delete or mod?

[djthom]

this is over my head...

where do i go to read up on this shit?!

good work 06speed6, i can't wait until i understand this!

[06Speed6]

Man I really need to reread the shit I type... I edited my second post.

Originally Posted by subparpunk03 My bad, I meant DYNAMIC compression ratio... so more increase in the intake cam means LESS dynamic compression... so we are running more increased VVT in the mid-load/part-throttle area, so wouldn't lowering the dynamic compression there be a good thing? and does the VVT increase the length of time that the intake cam is open, or just change the time at which the valve begins to open? If the valve opens earlier but stays open for the same length of time, I am picturing it in my head that increasing intake cam advance would increase the dynamic compression ratio... I'm still confuzzled.

Basic rule of thumb, higher LSA = higher rpm power and lower dynamic compression. So when the VVT adds 30* its significant.

The VVT doesnt increase the total amount of time the valve is open, it just moves its timing to a place that is more favorable to higher rpm power.

Im also saying advance as: moving the valve opening and closing event forward in the combustion event.

Originally Posted by phantom3 So if I understand this correctly, we could benefit from a VVT delete or mod?

Neither I think its pretty good as is, we just need to factor its existance into our idle and off idle timing and fueling tables.

Originally Posted by djthom this is over my head... where do i go to read up on this shit?! good work 06speed6, i can't wait until i understand this!

Buy a engine design book, the thicker it is and the more forumulas it has the better.

[subparpunk03]

OK, lets take a specific scenario. Here's what the VVT table looks like in ATR... we're running 30* of intake cam advance the second we get over 100% calculated load until we hit 4500 rpmsish.



That means we're opening the intake valves sooner when its more advanced, right? So that also means that the intake valves close earlier in the compression stroke, thus higher dynamic CR?

[06Speed6]

No they open and close later with higher advance. I guess I am using it the opposite of ignition advance.

I havent seen the VVT table before, I love it. Im going to look it over.

Stupid double post shit:
That chart is really counter intuitive, the VVT timing should advance in some linear fashion with rpm. Thats really weird shit.

[itzl0l]

so....since advancing the intake cam lowers DCR....(if i read right), adding a little advance in the areas we all see knock, (the lowerish loads in the 3k rpm area)....may help?


....potentially excited

[06Speed6]

Originally Posted by itzl0l so....since advancing the intake cam lowers DCR....(if i read right), adding a little advance in the areas we all see knock, (the lowerish loads in the 3k rpm area)....may help? ....potentially excited

According to the chart it only seems to advance timing in the low to mid RPMs, the exact opposite of what I expected.

Stupid load based crap, im going to need to think about this some more.

Perhaps we could lower the load threshold for cam advance, but the whole chart seems backwards.

[Lex]

Originally Posted by 06Speed6 According to the chart it only seems to advance timing in the low to mid RPMs, the exact opposite of what I expected. Stupid load based crap, im going to need to think about this some more. Perhaps we could lower the load threshold for cam advance, but the whole chart seems backwards.

Remember that this car stock falls on its face at high RPM so the 0 deg intake advance makes sense. The drop off in power at higher RPM is largely there for saving the catalytic converters.

The mid range intake advance on the other hand seems to mitigate a high DCR ... in an attempt to reduce knock in the meat of the torque band - but once again relying on stock restrictions.

Perhaps an increase in intake advance at lower loads is further needed when opening up the intake and exhaust.

Good info!

EDIT: Small FYI on LSA: http://74.125.155.132/search?q=cache...&ct=clnk&gl=ca

[itzl0l]

Originally Posted by Lex Remember that this car stock falls on its face at high RPM so the 0 deg intake advance makes sense. The drop off in power at higher RPM is largely there for saving the catalytic converters. The mid range intake advance on the other hand seems to mitigate a high DCR ... in an attempt to reduce knock in the meat of the torque band - but once again relying on stock restrictions. Perhaps an increase in intake advance at lower loads is further needed when opening up the intake and exhaust. Good info! EDIT: Small FYI on LSA: Technical Specs & Information

my thoughts exactly......

[06Speed6]

Just for shits and giggles, if we changed the direction of cam advance of a 108* LSA cam we get:

Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 78*
Inlet Valve Closes After Bottom Dead Center (ABDC): 39* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.81:1
Dynamic Cranking Pressure: 155.29 PSI
Dynamic Boost Compression Ratio: 19.6:1

Then the low and mid rpm, higher load parts of the chart would make less sence because the timing advance would make the LSA more agressive, but it would explain why our engines would blow.

[djuosnteisn]

Originally Posted by subparpunk03 OK, lets take a specific scenario. Here's what the VVT table looks like in ATR... we're running 30* of intake cam advance the second we get over 100% calculated load until we hit 4500 rpmsish. That means we're opening the intake valves sooner when its more advanced, right? So that also means that the intake valves close earlier in the compression stroke, thus higher dynamic CR?

Hmmmm, look at all dim zero's in the knock zone.



06speed6, muchos gracias for introducing me to this concept. Makes damn good sense!

[superskaterxes]

does anyone know our stock LSA? someone needs to start playing with this table to reduce low end power and open that shit up in the higher RPM's

[subparpunk03]

I'm still really struggling with this for some reason.

Is the following true:
Intake valves open at the tail end of the exhaust stroke
Stay open for the duration of the intake stroke
close slightly after the beginning of the compression stroke

Now, if you are running 0* advance or 30* advance the intake valves always remain open for the duration of the intake stroke and what changes is how long they are open during the exhaust and compression strokes. What you are saying is MORE advance means the intake valves are open for more of the compression stroke, and less advance means they're open for more of the exhaust stroke.

[superskaterxes]

on another MORE CRITICAL note whats the max and min values of this table and when will u begin to see interference? is there anyway to calculate this knowing valve length/travel?

[taf0422]

ohh this looks good..subscribing!

[djuosnteisn]

If the 0's in the table correspond to the higher DCR, i guess it could be feasible that mazda did it intentionally at the high rpm / high load areas, counting on the cooling characteristics of DI to keep detonation at bay.

It would also explain the knock at our infamous part throttle zone.

[AmillieMS6]

Originally Posted by nighthawk358 damn...we need more people like u 06speed6

we do.. his twin and long lost brother LEX

[djuosnteisn]

Oh and one more bit of input from me.

I've been playing with the VVT table and looking at my maf curve, and with lower VVT values at high load / high rpm, power (and maf g/s) seems to increase. It's just butt dyno data, but another possibly helpful tidbit. I don't recall my table going all the way to zero though. I'll check when i get home.

[06Speed6]

Originally Posted by subparpunk03 I'm still really struggling with this for some reason. Is the following true: Intake valves open at the tail end of the exhaust stroke Stay open for the duration of the intake stroke close slightly after the beginning of the compression stroke Now, if you are running 0* advance or 30* advance the intake valves always remain open for the duration of the intake stroke and what changes is how long they are open during the exhaust and compression strokes. What you are saying is MORE advance means the intake valves are open for more of the compression stroke, and less advance means they're open for more of the exhaust stroke.

Actually the lobe itself is unchanged, only the timing is changed, which means depending on the direction (+/-30*) then it would open later or earlier on the intake stroke.

I assumed that more advance is keeping it open for more of the compression stroke, but that should decrease overlap which would make the cam more favorable to high boost. Removing the advance would make it more favorable to n/a, low boost, and low rpms.

You know, I had this shit figured out in my head until you started talking

[Lex]

Also remember that there is a response time to VVT. It's not instantaneous.

Another note, 06speed6, are you sure on your definition of advance? Is advancing the intake cam same as advancing timing - in other words moving it further away from TDC or is it advancing it towards TDC?

[06Speed6]

I thought it would be advancing it towards the compression stroke TDC, not towards the exhaust stroke TDC. This is where the confusion is.

EDIT: Regardless of which direction, that chart still doesnt make even the slightest bit of sence. Because if we flip my definition of "advance" around, we would start with a 8.2:1 compression engine at low rpm/low load and move to ~8.8:1 compression for low rpm/high load, and we would maintain 8.2:1 compression for all high rpm loads.

If we use my definition of "advance", we start with 8.2:1 compression at low load/low rpm and move to progressively lower compression as load increases and according to the chart, retain 8.2:1 compression at high rpm for all loads.

[djuosnteisn]

I would imagine overlap would be desired in higher rpm, so lower values in the table correspond to intake valves opening earlier. Perhaps that table should be named VVT intake cam retard???

I think mazda was stuffing all those 0's in the PT KR zone of death for a reason.

And maybe after some breather mods, their coverage isn't enough. Maybe low numbers need to be expanded into the higher load areas.


eh, i don't know. Time to let the experts work.

[06Speed6]

Originally Posted by djuosnteisn I would imagine overlap would be desired in higher rpm, so lower values in the table correspond to intake valves opening earlier. Perhaps that table should be named VVT intake cam retard???

If the cam profile is unchanged and only LSA is changed, keeping the intake valve open around its max lift at the bottom of the intake stoke will put more air into the cylinder because the piston velocity is highest at TDC, second highest at BDC, and lowest in the middle on a inline type engine. So having the valve open longer at higher intake velocites helps with filling.

Also using that logic, moving the max lift area to the begining of the intake stroke and having it overlap more into the exhaust stroke would give more overlap but on a normal high overlap cam you would also have higher duration which is where the power is, I think we should have less total power like this because our duration is unchanged, but I could be wrong.

[superskaterxes]

ok heres the chart again from page 1



and heres a nearly stock dyno of MS6 (I/TP/BOV)




i would say the MEAT of the RPM with uber torque is in that fat range of 30's and 20's in the table. this would mean its SUBTRACTING from the LSA and u can also see HP peaks at the end where the table goes right to 0's.

[06Speed6]

I just took apart the VVT mechanism I have laying here, it advances towards the exhaust stroke which would be a lower LSA number.

Ironicly it also doesnt advance 30*, the max is 18* on my degree wheel.

Lets run the numbers again with this in mind...

Assume stock LSA is 108*, then:

Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 108*
Inlet Valve Closes After Bottom Dead Center (ABDC): 54* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.19:1
Dynamic Cranking Pressure: 141.89 PSI
Dynamic Boost Compression Ratio: 18.22:1

And with 18* max advance:

Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 90*
Inlet Valve Closes After Bottom Dead Center (ABDC): 45* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.58:1
Dynamic Cranking Pressure: 150.30 PSI
Dynamic Boost Compression Ratio: 19.09:1

Discuss in relation to the VVT chart.

[superskaterxes]

can u measure the stock LSA for us or no

[06Speed6]

I dont have a stock cam set, my aftermarket one is 115*, the stock evo is 108 or 110 I forget which.

115* is fairly high for a turbo cam except now that I know I can subtract 18* from it at idle... oh the possibilities.

With my cam it is:

Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 115*
Inlet Valve Closes After Bottom Dead Center (ABDC): 57.5* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.00:1
Dynamic Cranking Pressure: 137.81 PSI
Dynamic Boost Compression Ratio: 17.80:1

With -18* LSA...

Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 97*
Inlet Valve Closes After Bottom Dead Center (ABDC): 48.5* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.44:1
Dynamic Cranking Pressure: 147.28 PSI
Dynamic Boost Compression Ratio: 18.77:1

I still stay under the magic 8.5:1 ratio.

[FreeFlyFreak]

It seems there is some confusion regarding direction, this may help.

EFFECTS OF ALTERING CAMSHAFT TIMING
Advancing

Begins Intake Event Sooner
Open Intake Valve Sooner
Closes Intake valve Sooner
Builds More Low-End Torque
Decrease Piston-Intake Valve Clearance


Retarding

Delays Intake Closing Event
Keeps Intake Valve Open Later
Builds More High-RPM Power
Increase Piston-Intake Valve Clearance



Not sure which lowers Dynamic compression, I will leave it to you guys to sort that part out.

We cannot retard past 0, and cannot advance more than 30* I believe.

[06Speed6]

Advancing raises dynamic compression
Retarding lowers dynamic compression

We also only advance to 18*, not 30*.

[FreeFlyFreak]

Originally Posted by subparpunk03 That means we're opening the intake valves sooner when its more advanced, right? So that also means that the intake valves close earlier in the compression stroke, thus higher dynamic CR?

Correct

Originally Posted by 06Speed6 Advancing raises dynamic compression Retarding lowers dynamic compression We also only advance to 18*, not 30*.

In other words push the mid rpm zeros higher up in the load, and/or taper them more and/or lower max value.
Just for reference. I go into positive boost right around 84 load, 2-3 psi is right about 100, right where it hits max advance and max dynamic compression.

Funny thing is though for me (SRI and AP only), I dont get the partial throttle knock everyone talks about. All mine is WOT 4500-5700 unless I add octane booster to my Cali 91 octane. But I presume that is because I am pushing less psi than he fully bolted guys.

We also only advance to 18*, not 30*.

This is from the Cobb ATR help file:
"Table values are the degrees of timing advance (shown in camshaft degrees) the VVT system will
attempt to target. Higher values mean more intake camshaft advance, lower values mean less advance.
You cannot retard (use a value less than zero) the intake camshafts."
The max value I see in any table is 30*
Maye we are talking at cross purposes here, 30* from what??
Im guessing 30* from the unadvanced position, whatever that is, but it could be relative to something else. You are saying 18*, but 18* from what?, maybe we are talking the same thing really?

EDIT: I missed your post #31 ,so I dunno then??!!

[superskaterxes]

after talking to john from PTP the stock LSA is 103 or 104.

[Lex]

So with an LSA or 104, we have the following:

Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 104*
Inlet Valve Closes After Bottom Dead Center (ABDC): 52* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.28:1
Dynamic Cranking Pressure: 166.22 PSI
Dynamic Boost Compression Ratio: 18.42:1

And with 18* max advance:

Static Compression Ratio (SCR): 9.5:1
Lobe Seperation Angle (LSA): 86*
Inlet Valve Closes After Bottom Dead Center (ABDC): 43* (Lash not factored in)
Test Boost Pressure: 18psi
Altitude: Sealevel
Dynamic Compression Ratio: 8.67:1
Dynamic Cranking Pressure: 176.38 PSI
Dynamic Boost Compression Ratio: 19.29:1

[superskaterxes]

also your not going to be able to see a full 18 deg just because of the physical limitations of the VVT. also 18 deg is ALOT of overlap and you might even run into piston interference or start touching the exhaust valves.

[FreeFlyFreak]

Originally Posted by Lex So with an LSA or 104, we have the following: Static Compression Ratio (SCR): 9.5:1 Lobe Seperation Angle (LSA): 104* Inlet Valve Closes After Bottom Dead Center (ABDC): 52* (Lash not factored in) Test Boost Pressure: 18psi Altitude: Sealevel Dynamic Compression Ratio: 8.28:1 Dynamic Cranking Pressure: 166.22 PSI Dynamic Boost Compression Ratio: 18.42:1 And with 18* max advance: Static Compression Ratio (SCR): 9.5:1 Lobe Seperation Angle (LSA): 86* Inlet Valve Closes After Bottom Dead Center (ABDC): 43* (Lash not factored in) Test Boost Pressure: 18psi Altitude: Sealevel Dynamic Compression Ratio: 8.67:1 Dynamic Cranking Pressure: 176.38 PSI Dynamic Boost Compression Ratio: 19.29:1

So if I read that right....... at 18psi we would be better off zeroing that table out!!??
"Staying below 8.25 DCR is probably best for trouble free motoring" : for reference linky: http://www.empirenet.com/pkelley2/DynamicCR.html
Am I reading it right??? Beuller? Beuller? My brain hurts. I need booze......... it helps me think more clearly.
What are you guys using to calculate these numbers.

For my reference:

EFFECTS OF CHANGING LOBE SEPERATION ANGLE (LSA)
Tighten (smaller LSA number)

Moves Torque to Lower RPM

Increases Maximum Torque

Narrow Power band

Builds Higher Cylinder Pressure

Increase Chance of Engine Knock

Increase Cranking Compression

Increase Effective Compression

Idle Vacuum is Reduced

Idle Quality Suffers

Open Valve-Overlap Increases

Closed Valve-Overlap Increases

Natural EGR Effect Increases

Decreases Piston-to-Valve Clearance

from here: http://www.compcams.com/Technical/FAQ/LSAproperties.asp